pygame的两个对象渲染问题

我正在练习我的pygame知识，但是这里遇到了问题。我有两个矩形，但是当我运行程序时，其中一个正在闪烁。我尝试进行交换zombi.appear()，player.appear()并注意到闪烁的rect始终是第二个（因此在下面的情况下是player.appear()）。因此，我认为他们以某种方式相互干扰。请帮我解决这个问题。

提前致谢！

import pygame
import random

# Variables
inProcess = True
width, height = 500, 500
red = (255, 0, 0)
blue = (0, 0, 255)

# Pygame Settings
pygame.init()
screen = pygame.display.set_mode([width, height])
clock = pygame.time.Clock()


class Mob:
    def __init__(self, x, y, speed, type, color):
        self.x = x
        self.y = y
        self.speed = speed
        self.type = type
        self.color = color

    def render(self):
        pygame.display.flip()

    def appear(self):
        pygame.draw.rect(screen, self.color, (self.x, self.y, 64, 64))
        if self.type == 'player':
            self.move_player()
        if self.type == 'zombi':
            self.move_zombi()

        self.render()

    def move_player(self):
        keys = pygame.key.get_pressed()
        if keys[pygame.K_w] and self.y > 0:
            self.y -= self.speed
        elif keys[pygame.K_s] and self.y < height - 64:
            self.y += self.speed
        elif keys[pygame.K_a] and self.x > 0:
            self.x -= self.speed
        elif keys[pygame.K_d] and self.x < width - 64:
            self.x += self.speed

    def move_zombi(self):
        if self.y < 0 or self.y + 64 > height:
            self.speed *= -1

        self.y += self.speed


def main():
    player = Mob(64, 64, 1, 'player', red)
    zombi = Mob(256, 256, 2, 'zombi', blue)

    while inProcess:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                quit()

        screen.fill((255, 255, 255))

        zombi.appear()
        player.appear()

        clock.tick(60)


main()